Elliptic acetabular component for a hip prothesis

ABSTRACT

A hip cup for use as an acetabular component in a hip prosthesis including a shell part having a partially convex outer surface. The shell part is insertable in a cavity having an inner surface that is substantially defined by the outer surface of a segment of a sphere. The outer surface of the shell part substantially corresponds to the outer surface of a part of an ellipsoid and has an apex. During positioning of the shell part in the cavity, the shell part has a circumferential edge contacting the longitudinal edge of the cavity, with a space formed between the inner surface of the cavity and the apex of the shell part. A tool has an end releasably attachable to the shell part and a striking end. A handle on the tool permits positioning the tool and shell part to the cavity and striking the end to enact the shell in the part in a selected position.

BACKGROUND OF THE INVENTION

The invention relates to a hip cup for use as an acetabular component ina hip prosthesis, comprising a shell part having an at least partiallyconvex outer surface, which shell part can be inserted in a cavityhaving an inner surface that is substantially defined by the outersurface of a segment of a sphere.

The two techniques that are used most for attaching a hip cup concernthe cemented and the uncemented technique respectively. In the cementedtechnique, the hip cup is fixed by means of bone cement. Within theuncemented technique, screw hip cups and press-fit hip cups are used.Screw cups are screwed into a cavity, which may or may not have beenprovided artificially in a bone, press-fit hip cups are for instanceattached by knocking the hip cup into place in such cavity.

With these methods an attempt is made to approach the original anatomicsituation as much as possible through the proper positioning of theacetabular component in the acetabulum. The position of the press-fithip cup will change slightly in the first few months after positioning,due to the forces applied thereto. This change of position is commonlyreferred to as "settling".

For applying hip prostheses in patients, it is known to provide in anacetabulum a cavity having a truly spherical inner surface by means of aspherical cutter, wherein, subsequently, a press-fit hip cup isfittingly provided. Hence, this involves the outer surface of the hipcup abutting completely against the inner surface of the cavity formed,as a consequence of which, directly after the positioning of the hipcup, the forces acting on the hip cup are completely distributed on theinner surface of the cavity. The forces applied to the hip cup by thepatient, particularly during movements of his leg, will cause the hipcup to slightly change position, especially in the first few monthsafter positioning. Because the bone tissue around the hip cup will nothave healed entirely yet in those months, and the hip cup will hence nothave integrated entirely in the acetabulum yet, the hip cup will stillbe capable of moving somewhat, so that space will be created around thelongitudinal edge of the hip cup. As the hip cup rests against the apex,it cannot be pressed any further into the cavity and will thereforebecome increasingly looser due to a cranial displacement, with all itsconsequences, in particular with regard to the patient's mobility.Moreover, this will cause the occurrence of high local peak loads.

SUMMARY OF THE PRESENT INVENTION

Hence, the object of the invention is to provide a hip cup of the typedescribed in the opening paragraph, wherein the settling of the hip cupafter positioning remains possible in an advantageous manner, withoutinvolving the occurrence of the above-mentioned peak loads and changesof position. To this end, the hip cup according to the invention ischaracterized in that the outer surface of the shell part substantiallycorresponds to the outer surface of a part of an ellipsoid, thearrangement being such that during positioning, the shell part contactsthe longitudinal edge of the cavity at least by a circumferential edge,while a space is formed between the inner surface of the cavity and theapex of the shell part.

As the hip cup has an ellipsoid outer surface, it can be disposed in aspherical cavity with space, while a circumferential edge contacts thelongitudinal edge of the cavity. As a result, when the hip cup is loadedin the first few months after positioning, the hip cup canadvantageously be pressed somewhat further into the cavity, while thepart of the convex outer surface of the hip cup that is contacted withthe concave inner face of the cavity will become larger and larger. Dueto the nature of the change of shape of the cavity and the shape of thehip cup, the transmission of forces from the hip cup to the acetabulumwill largely be distributed on the periphery of the hip cup, as a resultof which load peaks are avoided. Because the hip cup according to theinvention has a flowing surface within the cavity, no tilting of the hipcup occurs, as it is guided along the inside of the cavity.

The elliptic form of the outer surface of the shell part has as a resultthat the space existing between the apex of the shell part and theinside of the cavity in the acetabulum is as small as possible, at leastin the final position, and will moreover be filled relatively quicklyand completely with the growth of new bone tissue, whereby an optimum,highly stable position of the hip cup in the acetabulum is obtained. Inmost cases, the anchoring of the hip cup in the acetabulum will hence beobtained entirely through press-fit, which has the advantage that noholes need to be drilled in the acetabulum and no screws need to bescrewed therein. After all, the danger of such holes and screws is thatin the case of an unfortunate location thereof, important nerves andveins and other vital parts of the body may easily be damaged, duringpositioning but also thereafter.

The part of the ellipsoid which defines the outer surface of the shellpart is preferably formed through rotation of an ellipse around theshort axis and the cutting of the thus formed ellipsoid approximatelyparallel to the circular section defined by the long axis of theellipse. Such form is of a simple construction and moreover enables aparticularly proper fit of the hip cup.

In a further elaboration of the hip cup according to the invention, theratio between the short axis and the long axis of the ellipsoid surfaceis approximately between 0.9 and 0.975, and is preferably 0.925. For theusual dimensions of the hip cups, this means that the height/sectionratio of the hip cup is optimal, while, moreover, the space is such thatsufficient movement is possible for a proper settling of the hip cup andthe space can still easily be filled with growth.

In a preferred embodiment of the hip cup according to the invention, theshell part is manufactured from titanium or a titanium-containing alloy,the outer surface of the shell part being at least partly porous. Thisembodiment has the advantage that the hip cup is biocompatible, so thatrejection reactions of the body do not take place, while the poroussurface enables the patient's bone tissue to partially grow in the outersurface of the hip cup, which further improves the adhesion between thehip cup and the acetabulum.

Preferably, the outer surface of the shell part is provided with acoating containing a calcium phosphate compound, preferably ahydroxyapatite. This will accelerate and further optimize the adhesionbetween the hip cup and the acetabulum.

The hip cup according to the invention preferably comprises an innershell, fittingly insertable substantially within the shell part, whichinner shell has an inner surface in the shape of a spherical shell andis manufactured from synthetic material, preferably from polyethylene.The inner shell forms a bearing surface for a hip head, to be rotatablyaccommodated therein, which hip head can be the natural as well as anartificial hip head. Because the inner shell is manufactured fromsynthetic material, good bearing properties with little friction andwear are obtained, and, moreover, a good damping of the forces appliedto the hip head is obtained.

The invention further relates to a mounting tool to be used forpositioning a hip cup in a cavity formed in an acetabulum, in particularsuitable for use in a hip cup according to the invention. The mountingtool according to the invention is characterized in that the toolcomprises a rod having at one end thereof an impact face and at theopposite end thereof fastening means, such as for instance screw thread,adapted for the positionally fixed, though detachable attachment of thehip cup to the tool, with a sleeve that can be fittingly arranged aroundthe rod, which sleeve comprises a locating grip extending at an anglerelative to the rod. With this mounting tool, the hip cup can beinserted in the cavity in an optimum manner through one or more blows onthe impact face, the sleeve with the handgrip permitting a properpositioning of the rod so that the hip cup is driven into the cavity inthe correct direction. As the hip cup can be fixedly mounted on the endopposite the impact face, the hip cup is prevented from changingposition during positioning. The handgrip is provided, preferably at theend remote from the impact face, with a counter impact face, allowingthe hip cup to be knocked out of the cavity as well by means of themounting tool.

The invention further relates to a method for positioning a hip cupaccording to the invention, wherein, with a spherical cutter, a cavityis formed having a substantially spherical inner surface, whereupon thehip cup is placed on a mounting tool and contacted, by a longitudinaledge thereof, with the longitudinal edge of the cavity in such a mannerthat the short axis is held in the proper position for positioning thehip cup, whereupon, subsequently, the hip cup is knocked into the cavityby means of the tool in such a manner that a space is left open betweenthe apex of the hip cup and the inner surface of the cavity, and theshell part of the hip cup, by a circumferential edge thereof, fixedlyabuts against an inner longitudinal edge of the cavity, whereupon,subsequently, the tool is removed and, finally, a suitable inner shellis inserted in the shell part in the desired position.

Hereinafter, to explain the invention, an exemplary embodiment of thehip cup will be described, with reference to the accompanying drawings.In these drawings:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a hip cup according to the invention,taken on the line I--I in FIG. 2;

FIG. 2 is a view of the shell part of the hip cup according to theinvention, from the open side;

FIG. 3 is a sectional view of an alternative embodiment of a hip cupaccording to the invention; and

FIG. 4 shows a mounting tool according to the invention.

The hip cup according to the invention, as shown in FIGS. 1 to 3,comprises an outer shell 1 and an inner shell 2, accommodated therein.The inner shell 2, preferably manufactured from synthetic material, suchas polyethylene, has a spherical inner space 3, wherein a hip head whichmay or may not be natural, not shown in the drawing, can be received toform a hip joint. The inner shell 2 is provided, along its outer surface4, with four evenly distributed lugs 5, which extend substantiallyradially from the outer surface and can each be received in one ofsixteen evenly distributed recesses 6 provided in the inner surface 7 ofthe outer shell 1. Along the outer longitudinal edge 8, the inner shell2 is provided with a stop shoulder 9 extending outside the outer shell 1and covering at least a portion of the lower edge 10 of the outer shell1.

The inner shell 2 can be fittingly accommodated in the outer shell 1such that the entire outer surface 4 of the inner shell 2 contacts theinner surface 7 of the outer shell, while the lugs 5 are tightlyaccommodated in the recess 6. The inner shell 2 can be disposed insixteen different positions relative to the outer shell 1, depending onwhich recesses 6 the lugs 5 are received in. This is particularlyimportant if the inner shell 2 is not symmetrical relative to all planespassing through the center line M, as is for instance the case in thestructural variant shown in FIG. 3, as will be further explainedhereinbelow.

The outer shell 1 has an ellipsoid outer surface 11, which outer surfaceis described by a body of revolution obtained through rotation of anellipse around the short axis K. In FIG. 1, this short axis coincideswith the center line M of the inner shell 2. The ratio of the short axisK relative to the long axis L of the ellipsoid is approximately between0.9 and 0.975, and preferably approximately 0.925. For hip cups of ausual dimension, for instance approximately 50 mm in section, this meansthat the short axis K is approximately 3.75 mm shorter than the longaxis L. Hence, in the dimensional example given, the maximum height ofthe outer shell 1 is approximately 1.875 mm smaller than the radius R ofthe circle described by the long axis L of the ellipsoid.

From the outer surface 11 of the outer shell 1, sixteen fins 18, evenlydistributed on the circumference, extend outward, which fins 18 have asomewhat sharp outer edge 19.

On the top side, and preferably in the apex 12 of the outer shell 1, anopening 13 is provided having an internal screw thread 14. Providedaround this opening 13 and on the inside of the outer shell 1 is a flat,annular recess 15, extending at right angles to the short axis K. Bymeans of the screw thread 14, the outer shell 1 can be mounted on amounting tool, as will be further explained hereinafter.

The mounting tool 50, as shown in FIG. 4, comprises a rod 51, having atone end 52 thereof an impact face 53 and a handgrip 54, and, at theother end 55 thereof, an outer screw thread 56 capable of cooperatingwith the screw thread 14 in the opening 13 in the hip cup. At the endremote from the impact face 53, the handgrip 54 comprises a counterimpact face 62. On the side of the screw thread 56 remote from theadjacent end 55 is a stop collar 60, at a distance from that end 55which approximately corresponds to the thickness of the outer shell 1 atthe level of the apex 12. The stop collar 60 has an outer diametercorresponding to the diameter of the annular recess 15 provided aroundthe hole 13 in the outer shell 1. Over the middle portion 57 of the rod51, a sleeve 58 can be arranged over the rod 51. The middle portion onwhich the sleeve can be arranged is bound on the one hand by thehandgrip 54 and on the other by a thickening 59 provided on the rod 51.From the sleeve 58, in the disposed condition, a locating grip 61extends in an inclined manner relative to the longitudinal axis of therod 51, in the direction away from the stop collar 60.

A hip cup according to the invention can be inserted in an acetabulum asfollows.

In an acetabulum 21 of a patient whose existing, natural hip cup is nolonger suitable for use, due to for instance a disease or trauma, aspherical cavity 16 is provided by means of a spherical cutter, and thesphere describing the inner face 17 of the cavity 16 has a center linecorresponding to the length of the long axis L. By means of the screwthread 14 in the opening 13, the outer shell 1 is fixedly screwed ontothe screw thread 56 on the end 55 of the rod 51, with the open side inthe direction of the rod 51. The outer shell 1 is screwed onto the rodfar enough for the stop collar 60 to contact the annular recess 15. As aresult, the outer shell is mounted on the end of the rod 51 in a fixedposition, allowing the outer shell 1 to be inserted in the cavity 16. Asa matter of fact, the outer shell can also be mounted on the rod bymeans of for instance clamping or like means.

By the side 22, facing away from the open side 20, of at least a numberof fins 18, the outer shell is contacted with the outer longitudinaledge 23 of the cavity 16, while the outer shell 1 is held in the properposition by means of the mounting tool 50. Subsequently, one or moreblows are given to the impact face 53 by means of a hammer or the like,in such a manner that the outer shell 1 is driven into the cavity, thefins 18 cutting into the acetabulum around the cavity 16. The rod 51 nowlies at least temporarily within the sleeve 58, brought into the desiredposition by means of the locating grip 61, which ensures that the outershell 1 is inserted in the cavity 16 in the correct position. In theembodiment shown in FIG. 1, this means that the short axis K of theouter shell 1 is parallel to the axis of symmetry of the cavity 16, butit is equally possible to arrange the outer shell 1 in a slightlyrotated position relative to this axis of symmetry, for instance for thepurpose of a positional correction, as shown in FIG. 3.

The outer shell 1 is driven into the cavity 16 far enough for the loweredge 10 thereof to be approximately level with the outer longitudinaledge 23 of the cavity 16, leaving a space 24 between the apex 12 of theouter shell 1 and the inner surface 17 of the cavity 16. Hence, theouter shell 1 contacts the inner surface 17 of the cavity 16 only by aportion 25 of the outer surface 11 which extends close to the lower edge10, with the fins 18 extending into the acetabulum 21, ensuring that theouter shell 1 does not rotate around the short axis K. Subsequently, themounting tool 50 is removed from the opening 13.

After the outer shell 1 has thus been disposed in the cavity 16 at theproper location and in the proper position, the inner shell 2 is pressedin the desired position into the outer shell 1, the lugs 5 beingreceived in the proper recesses 6, in such a manner that the inner shell2 is fixedly connected to the outer shell 1 and the hip cup is ready.Subsequently, the corresponding hip head can be received in the innerspace 3 of the inner shell 2.

Due to the load of the hip cup 1 in the first few months afterpositioning, and the fact that the bone tissue enclosing the cavity 16has not yet completely healed and grown in that period, the hip cup willbe pressed somewhat further into the cavity 16, the so-called settling.Because the outer surface 11 of the outer shell 1 is ellipsoid and thespace 24 is therefore left clear, the hip cup can be pressed furtherinto the cavity 16, the inner shape of the cavity 16 becoming slightlyellipsoid. By the outer surface 11, the outer shell 1 can simply slidealong the inner surface 17 of the cavity 16. As a result, the hip cupwill be fixed in the acetabulum 21 more properly. Moreover, initiallyafter positioning, a part of the forces applied to the hip cup can betransmitted by the caudal side of the hip cup to the acetabulum, asindicated by the arrow F in FIG. 1.

If the outer shell is truly spherical, as is the case in many known hipcups, this is not possible (or only to a very small extent), because theouter shell then has its apex abutting against the inner surface of thecavity, while along at least the caudal part of the longitudinal edgespace is created due to the (changing) load, causing an unfavorabledistribution of forces on the hip cup.

Due to the forces applied to the hip cup when the hip cup settles in thefirst few months after positioning, a slight yet relevant change ofshape of the cavity is effected, the inner surface of the cavitybecoming slightly ellipsoid. Consequently, the hip cup according to theinvention will fit increasingly better in the cavity.

In the first few months after positioning, the space 24 will be filledthrough the growth of bone tissue, which even further improves the fitof the hip cup. The outer shell 1 is manufactured from a titanium alloywith a porous top layer, and a coating is applied containing a calciumhydroxyapatite. Due to the coating, the bone growth toward the outershell 1 will be promoted, and the bone will grow slightly into theporous top layer. In addition, the outer shell 1 is sufficiently strongto withstand the forces applied thereto and is moreover biocompatible.

The inner shell 2 is manufactured from polyethylene, whichsimultaneously yields proper sliding properties and a damping action onthe forces to be transmitted by the hip head to the hip cup, as a resultof which high load peaks will be smoothed out.

In the structural variant of the hip cup shown in FIG. 3, the innershell 2' is provided with an antiluxation edge 26, projecting outsidethe outer shell 1' and approximately extending along the half of thecircumferential edge 9' of the inner shell 2' that is located on thecranial side. The antiluxation edge 26 increases in height in thedirection of the cranial side, and is intended to prevent a luxation ofthe hip head relative to the hip cup. Moreover, the inner shell 2' isprovided with the line of symmetry M' slightly inclined relative to theshort axis K of the outer shell 1', for instance for the purpose ofcorrecting the position of the maximum freedom of movement of thepatients leg. When such an inner shell 2' is used, it is particularlyadvantageous that it can be disposed in sixteen positions within theouter shell 1', because this permits a simple correction of smalldeviations of the position of the outer shell 1' relative to theacetabulum 21.

It will further be understood that, in principle, any number of recesses6 in the outer shell 1, 1' is possible, as long as the number at leastcorresponds to the number of lugs 5 on the inner shell 2, 2' and thedistribution thereof enables the lugs 5 to be received in the recesses6. For a stable positioning, it is desirable that at least two, butpreferably more than two lugs 5 are used, provided diametricallyopposite one another.

In the structural variant shown in FIG. 3, the outer shell 1' isprovided, on the cranial side thereof, with a recess 27, wherein thehead of a fixing screw to be used, optionally, can be received. Such afixing screw is intended to provide better stability of the hip cupwithin the acetabulum, and for this purpose, it is screwed through theouter shell 1' into a hole provided in the acetabulum 21. However, forthe great majority of uses of the hip cup according to the invention,such a fixing screw is superfluous, because due to the press fit, theouter shell 1, and hence the hip cup as a whole, can be mounted in theacetabulum 21 so as to be sufficiently stable.

If the hip cup should be removed from the cavity, for instance becauseof a correction to be carried out, the mounting tool 50 can be placedback in the opening 13, after which the hip cup can be knocked out ofthe cavity by striking the counter impact face with a hammer or a liketool.

The invention is not limited to the embodiments depicted in the drawingsand specification. Many other embodiments are possible within the scopeof the invention. For instance, other means can be used for anchoringthe outer shell against rotation, such as nails or screws driven intothe bone, differently shaped fins or, for instance, lugs or likeprojections provided in several rows over a substantial portion of theouter surface. Further, the inner shell can be designed in all sorts ofdifferent ways, depending on the desired possibilities of movement ofthe patient, and on the desired mounting method of the inner shell inthe outer shell. For instance, the center of rotation of the hip cupwithin the hip cup can be shifted by adjusting the shell shape of theinner shell, and the inner shell can be secured in the outer shell invarious manners other than the manner shown.

What is claimed is:
 1. A hip cup for use as an acetabular component in ahip prosthesis of a patient having a bone cavity including an innersurface configured as a segment of spherical surface and which includesa peripheral portion, said cup comprising an outer shell having apartially convex outer surface and a circumferential edge portion andhaving an apex and a central cross-section and forming one half of anellipse with a short radius through the apex and a long radius at theperipheral portion, the outer shell being configured for insertion intothe bone cavity,said circumferential edge portion of the outer surfaceof the outer shell upon insertion into said cavity contacts saidperipheral portion of the cavity and is configured to form a spacebetween the inner surface of the cavity and the apex of the outer shell,said outer surface of the outer shell being defined by rotation of saidellipse about said short radius and having a cross section in a planecontaining the long radius of the ellipse to form a half rotationalellipsoid, the ratio between the length of the short radius between saidapex and said long radius and the length of the long radius within theellipse is approximately between 0.9 and 0.975.
 2. The hip cup of claim1, wherein the hip cup comprises means (18) projecting outwardly fromthe shell surface, said means providing for the rotationally fixedattachment of the outer shell (1) in said bone cavity.
 3. The hip cupaccording to claim 1, wherein the outer surface (11) of the outer shell(1) is biocompatible and promotes osteo-integration over at least aportion of the outer surface.
 4. The hip cup according to claim 3,wherein the body of the outer shell (1) selected from the group oftitanium or a titanium-containing alloy.
 5. The hip cup according toclaim 3, wherein the outer surface (11) of the outer shell (1) is atleast partly porous.
 6. The hip cup according to claim 3, wherein theouter surface (11) of the outer shell (1) is provided with a coatingcontaining a calcium phosphate compound.
 7. The hip cup of claim 1comprising an inner shell (2) fittingly insertable substantially withinthe outer shell (1), said inner shell (2) having an inner surface in theshape of a spherical shell.
 8. The hip cup according to claim 7, whereinthe inner shell (2) is a synthetic material.
 9. The hip cup according toclaim 7, wherein the inner shell and said outer shell include means tosupport the inner shell in the outer shell in a number of relativerotational positions.
 10. The hip cup according to claim 7, wherein theinner shell comprises an antiluxation edge, said edge extending outsidethe outer shell.
 11. The hip cup of claim 1, wherein the outer shellincludes at least one opening having a fastening means for receiving amounting tool.
 12. The hip cup of claim 6, wherein said compound is ahydroxyapatite.
 13. The hip cup of claim 8, wherein said syntheticmaterial is polyethylene.